1. Field of the Invention
This invention relates to amplifier circuits, and in particular, to providing a lower distortion amplifier capable of creating high voltages to drive highly capacitive loads.
2. Description of the Prior Art
With the development of high performance audio formats like SACD and DVD-Audio, new demands are being placed on audio reproduction equipment. Electrostatic speakers provide the highest performance achievable to date. Most electrostatic speakers use conventional low voltage power amplifiers and step-up transformers to produce the thousands of volts necessary to drive electrostatic transducers. These output transformers limit the signal fidelity of what is achievable with electrostatic speakers.
Such a design needs to handle the difficult capacitive loading, yet provide extremely low distortion. Traditional high negative feedback amplifier designs have a very difficult time producing highly damped, low distortion signals at high currents while maintaining feedback control, in many applications. Large amounts of negative feedback have traditionally been used to control the varying current needs for the capacitance loading. Capacitive loading also increases the power handling demands on the amplifier at high frequencies.
The high fidelity requirements of such applications mandate that the level of distortion at all frequencies be imperceptibly small. At higher frequencies the dynamics of the music must be maintained while driving high current capacitive loads. At lower frequencies the current requirements are thousands of times smaller because of the higher impedances. With these extremes occurring simultaneously, the reproduced dynamics are much higher than just what is provided by the input signal. As a result, the amplifier must have much lower distortion than traditional high performance amplifiers, while handling extremely high impulse and high frequency currents cleanly. Therefore, the dynamic range and feedback systems of amplifiers for such applications need to be many times better than those designed for a traditional amplifier driving an almost pure resistive load.
One common characteristic of many high negative feedback amplifiers is that the distortion effectively increases, as the signal level gets lower. This is counter to the needs of an amplifier with dynamic range requirements of high capacitive loading. This phenomenon is the result of several processes. One process is the continual “hunting” that is inherent in these designs. The constant correction of the feedback circuit induces extra output noise into the output signal. This “hunting” signature of the output noise hides many of the subtle details of the input signal (e.g., the music signal, or an equivalent amplifier input signal). In audio amplifier applications, this output noise can mask the low and mid frequency music detail quite easily.
The large time delays of the large output devices required for the high currents in traditional amplifiers, and the delays present in the multiple input stages of the amplifier, can also produce noticeable overshoot and input stage overload, especially at high currents and high frequencies. This performance is needed in low distortion amplification of audio recording signals, and also for other applications requiring low signal distortion by amplifiers.
In view of the foregoing, what is needed is an improved method and circuit to provide the lowest distortion achievable by any amplifier and create high voltages to drive electrostatic speakers or other loads, without the limitations of using large output transformers driving highly capacitive transducer loads.